Thermometer

At the temperature change the electrical resistance of many electrically conductive materials changes as well. This resistance can be captured by the digital temperature measuring device and evaluated in relation to the temperature. The elements made of platinum and nickel are often used as resistance temperature detectors (RTD). As the temperature rises, their resistance increases and the conductivity decreases. Platinum shows a uniform change in resistance over a very wide temperature range. This enables platinum sensors for temperatures from - 200 to 1000 degrees Celsius. In contrast, temperature sensors made of nickel only have resistances that increase linearly with temperature for temperatures between - 60 and 300 degrees Celsius. Nickel sensors, however, react with higher sensitivity. Compared to platinum, they therefore show a greater change in resistance at the same temperature difference. In addition to the material, in the designation of the sensor there is also an indication of the value for the nominal resistance at zero degrees Celsius. A NI500 is therefore a nickel resistor with 500 Ohm nominal resistance and a PT1000 is a platinum resistor with 1000 Ohm nominal resistance. To avoid the measurement errors due to the influence of the long connecting cables on the measurement, the resistance sensors are also available with 3- and 4-wire technology.

Thermistors are temperature sensors based on semiconductors that are only electrically conductive under certain conditions. They are divided into NTC thermistors with a negative temperature coefficient and PTC thermistors with a positive temperature coefficient. The resistance of NTC thermistors decreases when the temperature increases. These NTC thermistors therefore conduct better at high temperatures, which is why they are also called thermistors. PTC thermistors, on the other hand, conduct better at lower temperatures. They are therefore PTC thermistors because their resistance increases with the temperature. In the digital temperature measuring device, both types of semiconductor sensors can be used for temperatures up to about 250 degrees Celsius. However, the thermistors can also be used as a component for automatic current limitation in electrically driven machines. In this case, the NTC thermistors are used to smoothly start the machines when they are switched on, and the PTC thermistors are used as overload protection, which reduces and finally blocks the current flow when the temperature rises too high.

Thermocouples consist of two wires of different metals connected at one end. If the unconnected ends have a different temperature than the connected ends, a voltage difference can be measured between two wires. To determine the temperature at the junction of the wires from this voltage difference, the temperature at the free ends must be known. There are more than 300 known material pairings for temperature measurement with thermocouples. Some of them, which are suitable for industrial measurements, have been standardised in DIN EN 60584. The thermoelectric voltages of these material combinations vary considerably from 5 mV/°C to 100 mV/°C. To simplify the use of the thermocouples, different types have been defined according to their properties: the types T, E, J, K and N as well as S/R, B, C and A for particularly high temperatures even above 1,000 degrees Celsius. The voltage measured at the thermocouple must be linearized in the temperature measuring device with the characteristic curve of the thermocouple used. If the temperature measuring device has connections for the standardised plugs of a certain type of the thermocouple, these thermocouples can be directly connected with the plug and used. If the compensating leads are required up to the measuring point, care must be taken that the compensating leads correspond to the type of the thermocouple.